The present invention relates in general to densitometers and, more particularly, to an optical system for a graphical display densitometer.
Densitometers are well-known as devices which scan a sample and provide an output signal for graphical display indicative of the optical density of the scanned sample. One well-known use of the densitometer is to scan a sample of blood which has been prepared by the electrophoresis process.
Electrophoresis of blood samples isolates various proteins in the blood, known as albumin, alpha-1 globulin, alpha-2 globulin, beta-globulin and gamma-globulin. The electrophoresis technique separates these proteins from each other and then the sample may be processed or scanned in a densitometer. Each of the proteins exhibits a different light absorption characteristic or pattern and the light absorption patterns are graphically displayed by the densitometer to indicate the presence and quantity of each protein.
In optical density analysis, the amount of light passing through the sample is an inverse logarithmic function of the optical density of the sample. The light transmitted through the sample falls on a photo-responsive element which generates electrical signals having a current proportional to the amount of transmitted light. The current output of the photo-responsive element is, therefore, also a logarithmic function of the optical density which then is converted into analog or time varying signals directly proportional to the optical density pattern of the scanned sample. The analog signals drive a graphic display unit to provide a permanent curve or record of the optical density pattern.
The analog signals, when graphically displayed, exhibit a series of peaks and valleys. In the analysis of blood, the area under the optical density curve and bounded by the two adjacent valleys separated by one peak, is representative of the quantity of each protein in the sample. Each point of the resultant curve is a function of the density of the sample at a corresponding position within the sample. It is a known technique to scan the sample and use an electronic integrator to quantitate the area under the curve and thereby determine the concentration of the sample. An example of a method and apparatus for graphic densitometer display is shown in U.S. Pat. No. 4,005,434 assigned to the assignee of the present invention.
In electrophoresis densitometry, both white light (visible mode) and ultraviolet energy sources (fluorescent mode) are used. A tungsten lamp may be used for transmission densitometry (i.e., using white light) because it emits useful amounts of radiation over the visible spectrum. A low pressure mercury source may be used for the fluorescent mode of operation because it has a strong emission at 366 nm, a wave length efficient for exciting most fluorescent samples. Unlike transmission densitometery, the ultraviolet energy is used to excite the sample. The light emitted by the sample is at a wave length different that the ultraviolet energy. In the fluorescent mode, the light detected and measured is that emitted by the sample.
While a photocell may be used as the detector in the visible mode, it is not satisfactory in the fluorescent mode because the intensity of the fluorescence is low in most cases. Thus, a photomultiplier tube or equivalent type detector is used because it has the high sensitivity necessary to detect weak fluorescence. The photomultiplier tube detector may also be optimized for operation in the visible spectrum.
Filters are required in both the visible and fluorescent modes of operation when using a photomultiplier tube in order to protect the detector. For fluorescent densitometry, the ultraviolet source is placed on the same side of the sample as the detector because otherwise some of the support media would block or filter ultraviolet light and the sample would not be excited if the light were on the opposite side. Because of this arrangement of source and sample for fluorescent densitometry, a filter is required to block ultraviolet energy from the exciting source below a certain level from reaching the detector. Further, the photomultiplier detector is so sensitive that in the visible mode, an optical density filter may be necessary to protect the photomultiplier detector if the sample is not dense enough such that the detector will remain in the linear region of operation and not become saturated. Thus, several types of filters are required when transmission and fluorescent densitometry are incorporated in one machine and when a single sensitive detector such as a photomultiplier tube is used.
A problem has existed in efficiently handling the plurality of filters required for the fluorescent and visible modes while insuring that the filter selected matches the particular mode of operation. A typical method of operation has been that the operator manually changes the filters depending upon whether transmission or fluorescent densitometry is desired. This is unacceptable because the manual handling of the filters adversely affects the quality of the detected light. Further, the filters may become misplaced or damaged thereby making them unavailable when desired. Thus, the present optical system has been devised such that the operator may conveniently select a desired filter for either mode of operation, and the filters are always available to be used when needed.